AS suggested.....two ways for fresh water tank to get filled when connected to 'city water'
IF you have a water panel where hose connects than check the valving...fill tank valve open.
Otherwise the on-board pump internal check valve is letting water run thru pump back into tank
The internal check valve can get build up on it from hard water or debris from tank holding it open.
Running the pump can clean out the debris (and lower the fresh water tank).
Cleaning the check valve is easy....just remove pump head and clean diaphragms
OR replace with new.